Barba Ibanez Osmar's second-half penalty ensured that United took the three points at the Saraburi Stadium and clinched their third successive triumph.
It also allowed Buriram to take advantage of Muang Thong United's 2-2 draw at Police United, in which Michael Murcy equalised for the hosts three minutes from time.
The visitors had taken the lead at the Thammasat Stadium through Dagno Siaka before Chompon Buangam equalised.
However, it looked as though Mario Gjurovski had won it for the away side but, after 87 minutes, French forward Murcy snatched a point for his side.
Third-place Chonburi picked up only their second win in five games against Chainat, winning 2-1 after surviving a late scare when Yannick Mbengono scored in the 90th minute.
However, goals from Leandro Assumpcao and Jetsadakorn Hemdaeng earned them the three points and moved them seven points clear of Suphanburi in fourth after Phayong Khunnaen's men were beaten 1-0 by Pattaya United.
Pairote Sokam's 90th minute own-goal gave Pattaya a surprise win and they now sit level on points with Samut Songkhram after their match with Chiangrai United was abandoned due to crowd trouble.
Ratchaburi's hopes of moving further clear of the drop zone were dealt a blow with a narrow 1-0 defeat to Bangkok Glass.
Bangkok United inflicted the eighth consecutive defeat on TOT, winning 2-1, while Songkhla United's poor form also continues.
They were beaten 2-0 by BEC Tero Sasana on Saturday, meaning they are without a win in seven games.
